1. Field of the Invention
The present invention relates in general to a system for controlling a steering angle of a vehicle such as a motor vehicle, and more specifically to a system for controlling the steering angle of secondary steering wheels of a motor vehicle having four steering wheels, in relation to the steering angle of primary steering wheels of the vehicle.
2. Discussion of the Related Art
In a 4-wheel-steerable (4-WS) motor vehicle having two front steering wheels and two rear steering wheels, it is generally recognized that the front steering wheels serve as primary steering wheels while the rear steering wheels serve as secondary or auxiliary steering wheels. In a steering control system for such 4-wheel-steerable vehicle, a mechanical actuator controllable by the vehicle driver through a steering shaft is used to control the steering angle (hereinafter referred to as "primary steering angle) of the primary steering wheels, while an actuator in the form of an electrically operated motor is used to control the steering angle (hereinafter referred to as "secondary steering angle") of the secondary steering wheels such that the actual secondary steering angle as detected by a sensor coincides with a desired or target value which is determined on the basis of or in relation to the primary steering angle. In other words, the motor is electrically controlled so as to zero a difference between the determined target value and detected value of the secondary steering angle.
It is known that the speed of turning (generally referred to as "yaw velocity or yaw rate) of the vehicle about a vertical line which passes the gravity center of a motor vehicle increases with a rate of change in the steering angle. Consequently, a relatively rapid change in the rate of change in the primary steering angle will cause deterioration of the steerability of the vehicle and transverse slipping of the vehicle in a direction normal to the direction of running of the vehicle. JP-A-59-100062 discloses a steering control system adapted to control the secondary steering angle in relation to the yaw rate (yaw velocity) of the vehicle detected by a yaw rate sensor. However, this publication is silent on the details of a specific manner in which the secondary steering angle is controlled. JP-A-60-161256 teaches the use of a yaw rate gain Ys/.theta., namely, a ratio of the yaw rate to the steering angle .theta., which changes with a steering frequency (Hz) such that the yaw rate gain Ys/.theta. is the highest at 1 Hz and continuously decreases as the steering frequency increases or decreases from 1 Hz. The latter publication further teaches the desirability of maintaining the yaw rate gain (Ys/.theta.) constant in order to assure high steerability or stable steering of the vehicle, and at the same time either increasing a control gain K1 which determines an amount of change in the secondary steering angle with respect to the detected yaw rate, as the vehicle speed increases, or changing the control gain K1 according to a command generated by manipulation of a switch by the operator of the vehicle.
JP-A-60-124572 shows a secondary steering angle control system in which a target yaw rate is determined or calculated from detected primary steering angle S and vehicle speed F, and the actuator for changing the secondary steering angle is controlled so that the actual yaw rate detected by a yaw rate sensor coincides with the determined target yaw rate. However, JP-A-60-124572 does not teach details on the method by which the target yaw rate which suits the running condition of the vehicle is determined from the detected primary steering angle S and vehicle speed F. JP-A-63-192667 points out that the feedback control of the secondary steering angle on the basis of the yaw rate as proposed in the above-identified publication JP-A-60-124572 suffers from a time delay in the detection of the yaw rate with respect to the moment of a change in the actual yaw rate, and cannot be an effective measure for improving the steerability of the vehicle. In the light of this drawback, JP-A-63-192667 proposes an improved feedback control adapted to deal with the detection delay of the yaw rate.
In the secondary steering angle control system of the type in which the secondary steering angle is controlled according to the determined target yaw rate value, the secondary steering angle cannot be suitably controlled upon occurrence of any abnormality in the detected yaw rate, namely, in the presence of a relatively large error or noise in the output of the yaw rate sensor. For fail-safe control of the secondary steering angle, it may be a considered arrangement to determine whether the output signal of the yaw rate sensor has an intolerable error or not, and zero the secondary steering angle if the output signal is found to have an error larger than a threshold. This arrangement, however, suffers from an abrupt change (rapid zeroing) in the secondary steering angle in the event of detection of such excessive error in the output of the yaw rate sensor, whereby the running direction of the vehicle is likely to be changed unexpectedly to the vehicle driver. In other words, the forced zeroing of the secondary steering angle upon detection of abnormality in the detected yaw rate sensor will cause reduction in the steerability of the vehicle, with the secondary steering wheels kept at their neutral position in spite of a change in the primary steering angle.
The above drawback may be encountered in the control of the steering angle of the vehicle in general, irrespective of whether the steering angle is that of the front steering wheels or that of the rear steering wheels, provided that the detected yaw rate of the vehicle is utilized to control an electrically controlled actuator for controlling the front or rear steering angle of the vehicle.